(1) Technical Field
The present invention is related to power supply control devices and image forming apparatuses. Specifically, the present invention is related to a technology of preventing destruction of a power supply control device upon cold start without extended warm-up time.
(2) Description of the Related Art
Image forming apparatuses utilizing electrophotography have heat sources for heat-fixing toner onto recording sheets. Among various types of such heat sources, halogen heaters are widely used for achieving low cost. Fast switching of power supply to halogen heaters can be achieved by using step-down chopper circuits.
However, halogen heaters may be problematic for inducing high inrush current, particularly immediately after they are turned on from a cold state (an off state with low halogen heater temperature). This is because halogen heaters have low electrical resistance in the cold state.
The flow of high inrush current through a halogen heater results in voltage drop of a power supply line connected to the halogen heater and increases the risk of consequent problems such as flickering of fluorescent lamps connected to the power supply line.
Further, particularly when a step-down chopper circuit connected to a halogen heater includes an insulated gate bipolar transistor (IGBT) with low withstand voltage, turning on the halogen heater in the cold state (referred to in the following as a cold start of a halogen heater) at a high duty cycle may result in destruction of the IGBT.
While IGBT destruction by inrush current can be prevented by using an IGBT with high withstand voltage, this measure is not practical because the use of an IGBT with high withstand voltages is costly.
Another measure for preventing IGBT destruction by inrush current is reducing the pulse-width modulation duty cycle initially used following the cold start of a halogen heater, and thereby suppressing inrush current. However, the longer the reduced duty cycle is used, the longer the warm-up time of the halogen heater.
In view of such problems, a conventional technology has been proposed, for example, of restricting input voltage of a lamp for a predetermined amount of time from when the lamp is turned on so that the current flowing through the lamp has a value smaller than a permissible current value of the lamp, and commencing pulse-width modulation after elapse of the predetermined time amount. One example of this conventional technology can be found disclosed in Japanese Patent Application Publication No.: 2001-069667.
Restricting input voltage by employing such a configuration suppresses inrush current and thereby prevents IGBT destruction.
Meanwhile, one of the benchmarks used to assess the performance of image forming apparatuses is the amount of time required for an initial page to be printed and output from when a user instruction for printing is received. This amount of time is referred to as first copy output time (FCOT), and the shorter the FCOT, the more desirable.
However, with the conventional technology described above, input voltage is always restricted for a predetermined amount of time every time the halogen lamp is turned on, and due to this, a long amount of time is always required for startup. That is, applying the conventional technology described above to an image forming apparatus results in an extension in the amount of time required for warm up and a consequent extension in FCOT, due to the conventional technology requiring restricting input voltage for a certain amount of time.